This is the 7118th most frequent Russian word.
борисевич
"Борисевич" is a patronymic surname or middle name in Russian, meaning "son of Boris."
In this sentence, 'Борисевичем' refers to a surname used in the instrumental case, indicating who the speaker was conversing with.
Вчера я разговаривал с Борисевичем о проекте.
Yesterday, I spoke with Borisevich about the project.
Here, 'Борисевич' is used as a nominative proper noun, indicating the subject of the sentence.
Борисевич всегда любил помогать людям.
Borisevich has always loved helping people.
In this case, 'Борисевичу' represents the dative case of the name, showing to whom the letter was handed.